1. Field of the Invention
The present invention relates to a power supply control apparatus (referred to in the following simply as a control apparatus) for controlling the supply voltage of an induction motor, and in particular to an induction motor control apparatus which provides a high efficiency of operation.
2. Description of the Related Art
In the prior art, there have been proposals for a type of induction motor control apparatus whereby the power factor at which an induction motor operates is detected, and a feedback control loop is utilized to control the level of supply voltage of the induction motor such as to maintain the operating power factor close to a target value, i.e. a value which will ensure a high degree of operating efficiency. Such a type of apparatus can be effective if the variations in load torque applied to the induction motor occur in a relatively gradual manner. However in the case of an induction motor which is subjected to abrupt changes in load torque, and in particular with abrupt changes from a light load to a high load condition occurring, it is found that control by such a prior art type of apparatus is ineffective, so that the-induction motor cannot follow the changes in applied load. In some cases, if a sudden increase in load torque occurs, and the control apparatus cannot increase the supply voltage of the motor sufficiently rapidly to counteract the increased load, then the motor may stall.
If it is attempted to modify such a prior art type of induction motor control apparatus such that the motor supply voltage will change more rapidly, i.e. such that the supply voltage can closely follow a sudden increase in motor load, it is found that the control apparatus becomes unstable, and oscillation may occur. Thus, satisfactory control cannot be achieved. It is thus necessary to incorporate an integrator circuit into the aforementioned feedback control loop, to reduce the rate at which the motor supply voltage can change. Hence the above problem cannot be overcome, i.e. the induction motor cannot follow a sudden change in the applied load torque. If there is a sudden increase in the load torque which exceeds the drive capability of the motor (i.e. the drive capability provided by the level of motor supply voltage that is currently being applied) then a rapid increase in the degree of motor slip will occur. As a result, there will be a sudden drop in the value of motor power factor that is detected by the control apparatus. When that occurs, then in spite of the fact there is a need for the motor supply voltage to be increased, the control apparatus will actually function to lower the motor supply voltage, so that the motor may stall.